EXTREME PHOSPHATE DEFICIENCY DECREASES THE IN-VIVO CO2/O2 SPECIFICITY FACTOR OF RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE-OXYGENASE IN INTACT LEAVES OF SUNFLOWER

Sunflower plants were grown under controlled environmental conditions with either 0 or 10 mol m-3 phosphate (Pi). From steady-state measurements of gas exchange and chlorophyll fluorescence made on intact leaves, the in vivo CO2/O2 specificity factor (in vivo K(sp)) of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) was determined following two methods based on models of C3 photosynthesis by Brooks and Farquhar (I 985) and Peterson (1989). The two methods gave in vivo K(sp) values for control sunflower leaves which were similar to published values for higher plants. Extreme Pi deficiency decreased in vivo K(sp) in sunflower leaves compared to adequate Pi. This suggests that Pi deficiency affected photorespiration less than photosynthesis. The decrease in in vivo K(sp) may be due to a real change in the enzyme kinetics favouring oxygenation more than carboxylation or due to an increase in the number of CO2 molecules released per oxygenation; in which case the observed decrease in the in vivo K(sp) determined on intact leaves will not agree numerically with the true K(sp) of Rubisco determined in vitro using purified enzyme from the same leaf. We discuss the implications of the relatively large photorespiration in Pi-deficient sunflower leaves with respect to the increased dissipation of photosynthetic electrons and photorespiratory recycling of Pi in the chloroplast stroma. Although our results on in vivo K(sp) suggested a relatively larger photorespiratory potential in Pi-deficient than control sunflower leaves, photosynthesis was insensitive to O2 in Pi-deficient leaves; the possible reasons for this phenomenon are discussed. Under extreme Pi deficiency, O2 sensitivity of photosynthesis is not a reflection of the in vivo photorespiratory rates. Determination of in vivo K(sp) of Rubisco is a useful approach to study the photorespiratory potential of intact leaves.